AT&T will let you unlock your device permanently, provided your contract has already expired. AT&T customers still on contract may unlock their devices up to five times per year (for international travel purposes), as long as they don’t owe any past-due amount on their accounts, and have been a customer for 60 days or more.
Verizon sells all iPhone 5s unlocked, meaning you could take your device over to AT&T or T-Mobile without having to unlock the device. But other devices, like the Samsung Galaxy S3, are locked by Verizon. However, like AT&T, customers in good standing can request to have their devices unlocked for international travel.
Sprint does not offer unlocked devices, but it will allow customers to unlock their devices for travel after three months with the carrier.